igb: Fix XDP with PTP enabled

When using native XDP with the igb driver, the XDP frame data doesn't point to
the beginning of the packet. It's off by 16 bytes. Everything works as expected
with XDP skb mode.

Actually these 16 bytes are used to store the packet timestamps. Therefore, pull
the timestamp before executing any XDP operations and adjust all other code
accordingly. The igc driver does it like that as well.

Tested with Intel i210 card and AF_XDP sockets.

Fixes: 9cbc948b ("igb: add XDP support")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>